The present invention relates to pellicles that are used in photolithography operations, and more particularly to a method for manufacturing optical pellicles.
In the semi-conductor chip industry it is well known that pattern transfer from the photomask to substrate is accomplished by exposing a mask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the photomask are projected onto the substrate which has been treated with a photosensitive substance. This results in the mask etching being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on the mask surface, such that the pellicle membrane extends parallel to the mask at a predetermined distance spaced away from it. Any contamination which would ordinarily land on the mask surface instead falls onto the pellicle membrane.
Pellicles substantially eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The use of pellicles can increase the quality of the resulting circuit, thereby dramatically improving circuit fabrication productivity. Consequently, it is no surprise that pellicle manufacturing techniques have become increasingly important because high quality pellicles are critical to the success of the photolithographic process.
During the pellicle manufacturing process, it is important to minimize the possibility of either relatively large or small contaminant particles being deposited on the pellicle membrane. Relatively large particles are unacceptable because they may be reproduced in the substrate during photolithography even though they are out of focus. Equally unacceptable are particles (whether large or small) that are deposited on the underside of the pellicle membrane or on the pellicle frame. Such particles may drop onto the mask surface during photolithography which is precisely what is to be avoided by using pellicles.
It is also critical that the pellicle membrane be extremely uniform across its surface. This is so that the light passing through the membrane during lithography is unobstructed and is not in any way refracted. Thus, the composition of the membrane must be highly uniform, and the membrane must be evenly tensioned across the pellicle frame. Also, it is important to ensure that a continuous seal exists between the thin film and the frame.
To further understand these important requirements, it is necessary to provide an explanation as to how pellicles are formed. As is known in the art, forming a free-standing membrane is the first step in pellicle manufacture. Commonly the membrane is prepared by spinning a suitable polymer, such as nitrocellulose or some other cellulose-based polymer, on a substrate. The newly formed membrane is then removed from the substrate and held under tension, adjacent its periphery, to prepare it for subsequent manufacturing steps.
Next, a frame is fastened, or bonded, to a working area of the membrane, framing the working area. Currently, this fastening step involves applying an adhesive to a pellicle frame, followed by mounting the pellicle membrane onto the frame. When applying the adhesive, it is important that the adhesive is applied with a uniform bead in a very thin layer. Some existing manufacturing methods utilize an adhesive dispenser which contacts the pellicle frame when applying the adhesive. Others maintain a spaced relationship between the dispenser and the frame. In both of these methods, inspection of the adhesive dispensing process is very difficult.
In existing methods which utilize an adhesive dispenser that contacts the pellicle frame, the dispenser tip is generally selected to be either rigid or flexible. The problem with rigid dispenser tips which contact the pellicle frame is that they are prone to leaving scratches or marks on the pellicle frame. These marks generally remain, even if the pellicle is subsequently polished or lapped. The applied adhesive may also flow into the scratches, resulting in non-uniform and overly broad spreading of the adhesive. Rigid dispenser tips may also become worn from contacting the pellicle frame. This deformation results in non-uniform dispensing of the adhesive. Flexible dispenser tips are less likely to leave dispensing marks or scratches on the frame, but they are more easily worn out and deformed.
Manufacturing methods which maintain a spaced relationship between the adhesive dispenser and the pellicle frame present different problems. One primary problem is controlling the dispensing gap between the pellicle frame and the adhesive dispenser. This problem is exacerbated by the fact that not all pellicle frames are of the same thickness.
Some existing methods of producing pellicles involve hand-dispensing the adhesive onto the pellicle frame. The problem with hand-dispensing methods is that it is very difficult to achieve the required uniform, thin layer of adhesive. Acetone may be used to dilute the adhesive, but this raises the additional problem that it is hard to control the rate of evaporation of acetone.
A further problem with existing methods arises if any oil is present on the pellicle frame. This oil reduces the affinity between the applied adhesive and the pellicle frame, resulting in a broader, non-uniform layer of adhesive.
Accordingly, it is a general object of the present invention to provide a new method for manufacturing optical pellicles which avoids the drawbacks and limitations of the prior art methods. More specifically, the invention has an object to develop a manufacturing method which provides an improved system for mounting the free-standing pellicle membrane to the pellicle frame. It is a further object of the present invention to develop a pellicle manufacturing method which allows easy inspection of the adhesive dispenser.
Still another object of the present invention is to provide a method of applying adhesive for bonding the pellicle membrane to the pellicle frame, which is readily suited to being a part of a highly automated, easily repeatable, process. Yet another object of the present invention is to provide a method for manufacturing optical pellicles in which the pellicle membrane can be very precisely positioned on a pre-cut pellicle frame.